Staking Reward Rate Runtime API

bin/node/runtime/src/constants.rs

@@ -30,6 +30,17 @@ pub mod currency {
 	}
 }
 
+/// Inflation parameters.
+pub mod inflation {
+	use sp_runtime::Perquintill;
+	// NOTE: Changes to these parameters must also be applied in
+	// `pallet_staking_reward_curve::build!`.
+	pub const IDEAL_STAKE_BASE: Perquintill = Perquintill::from_percent(75);
+	pub const FALLOFF: Perquintill = Perquintill::from_percent(5);
+	pub const MIN_INFLATION: Perquintill = Perquintill::from_perthousand(25);
+	pub const MAX_INFLATION: Perquintill = Perquintill::from_percent(10);
+}
+
 /// Time.
 pub mod time {
 	use node_primitives::{BlockNumber, Moment};

bin/node/runtime/src/lib.rs

@@ -101,7 +101,7 @@ use impls::{AllianceProposalProvider, Author, CreditToBlockAuthor};
 
 /// Constant values used within the runtime.
 pub mod constants;
-use constants::{currency::*, time::*};
+use constants::{currency::*, inflation::*, time::*};
 use sp_runtime::generic::Era;
 
 /// Generated voter bag information.
@@ -538,6 +538,8 @@ impl pallet_session::historical::Config for Runtime {
 	type FullIdentificationOf = pallet_staking::ExposureOf<Runtime>;
 }
 
+// NOTE: Changes to `min_inflation, `max_inflation`, `ideal_stake` and `falloff` must also be
+// applied in constants::inflation.
 pallet_staking_reward_curve::build! {
 	const REWARD_CURVE: PiecewiseLinear<'static> = curve!(
 		min_inflation: 0_025_000,
@@ -2101,6 +2103,9 @@ impl_runtime_apis! {
 	}
 
 	impl pallet_staking_runtime_api::StakingApi<Block, Balance> for Runtime {
+		fn reward_rate() -> Perquintill {
+			Staking::api_reward_rate(MIN_INFLATION, MAX_INFLATION, IDEAL_STAKE_BASE, FALLOFF)
+		}
 		fn nominations_quota(balance: Balance) -> u32 {
 			Staking::api_nominations_quota(balance)
 		}

frame/staking/Cargo.toml

@@ -28,6 +28,7 @@ pallet-session = { version = "4.0.0-dev", default-features = false, features = [
 	"historical",
 ], path = "../session" }
 pallet-authorship = { version = "4.0.0-dev", default-features = false, path = "../authorship" }
+pallet-staking-reward-fn = { version = "4.0.0-dev", default-features = false, path = "./reward-fn" }
 sp-application-crypto = { version = "7.0.0", default-features = false, path = "../../primitives/application-crypto" }
 frame-election-provider-support = { version = "4.0.0-dev", default-features = false, path = "../election-provider-support" }
 log = { version = "0.4.17", default-features = false }
@@ -62,6 +63,7 @@ std = [
 	"sp-runtime/std",
 	"sp-staking/std",
 	"pallet-session/std",
+	"pallet-staking-reward-fn/std",
 	"frame-system/std",
 	"pallet-authorship/std",
 	"sp-application-crypto/std",

frame/staking/runtime-api/Cargo.toml

@@ -15,10 +15,12 @@ targets = ["x86_64-unknown-linux-gnu"]
 [dependencies]
 codec = { package = "parity-scale-codec", version = "3.0.0", default-features = false, features = ["derive"] }
 sp-api = { version = "4.0.0-dev", default-features = false, path = "../../../primitives/api" }
+sp-runtime = { version = "7.0.0", default-features = false, path = "../../../primitives/runtime" }
 
 [features]
 default = ["std"]
 std = [
 	"codec/std",
 	"sp-api/std",
+	"sp-runtime/std"
 ]

frame/staking/runtime-api/src/lib.rs

@@ -20,12 +20,17 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
 use codec::Codec;
+use sp_runtime::Perquintill;
 
 sp_api::decl_runtime_apis! {
+	#[api_version(2)]
 	pub trait StakingApi<Balance>
 		where
 			Balance: Codec,
 	{
+		/// Returns the current staking reward rate.
+		fn reward_rate() -> Perquintill;
+
 		/// Returns the nominations quota for a nominator with a given balance.
 		fn nominations_quota(balance: Balance) -> u32;
 	}

frame/staking/src/pallet/impls.rs

@@ -35,7 +35,7 @@ use frame_system::{pallet_prelude::BlockNumberFor, RawOrigin};
 use pallet_session::historical;
 use sp_runtime::{
 	traits::{Bounded, Convert, One, SaturatedConversion, Saturating, StaticLookup, Zero},
-	Perbill,
+	Perbill, Perquintill,
 };
 use sp_staking::{
 	offence::{DisableStrategy, OffenceDetails, OnOffenceHandler},
@@ -974,6 +974,46 @@ impl<T: Config> Pallet<T> {
 }
 
 impl<T: Config> Pallet<T> {
+	/// Returns the current staking reward rate.
+	pub fn api_reward_rate(
+		min_inflation: Perquintill,
+		max_inflation: Perquintill,
+		ideal_stake: Perquintill,
+		falloff: Perquintill,
+	) -> Perquintill {
+		match ActiveEra::<T>::get() {
+			Some(active_era) => {
+				let staked = ErasTotalStake::<T>::get(active_era.index);
+				let total_issuance = T::Currency::total_issuance();
+				let staked_percent = Perquintill::from_rational(staked, total_issuance);
+
+				// NOTE: The JavaScript implemenation considers 2 parameters, `auctionMax` and
+				// `auctionAdjust` in the following calculation:
+				//
+				// Math.min(auctionMax, numAuctions.toNumber()) * auctionAdjust;
+				//
+				// `auctionMax` is always 0 on PJS Apps and Staking Dashboard. This calculation is
+				// therefore being ignored here for now.
+				let ideal_interest = max_inflation / ideal_stake;
+				let curve = if staked_percent <= ideal_stake {
+					(ideal_interest.saturating_sub(min_inflation / ideal_stake))
+						.saturating_mul(staked_percent)
+				} else {
+					let curve_base =
+						ideal_interest.saturating_mul(ideal_stake).saturating_sub(min_inflation);
+					let curve_power = (staked_percent - ideal_stake).int_div(falloff);
+					let curve_multiplier = Perquintill::from_percent(1) /
+						Perquintill::from_percent(2u64.pow(curve_power as u32));
+					curve_base.saturating_mul(curve_multiplier)
+				};
+
+				let reward_rate = min_inflation + curve;
+				reward_rate
+			},
+			None => Perquintill::zero(),
+		}
+	}
+
 	/// Returns the current nominations quota for nominators.
 	///
 	/// Used by the runtime API.

frame/staking/src/tests.rs

@@ -398,6 +398,21 @@ fn rewards_should_work() {
 	});
 }
 
+#[test]
+fn api_reward_rate_works() {
+	ExtBuilder::default().build_and_execute(|| {
+		assert_eq!(
+			Pallet::<Test>::api_reward_rate(
+				Perquintill::from_perthousand(25),
+				Perquintill::from_percent(10),
+				Perquintill::from_percent(75),
+				Perquintill::from_percent(5)
+			),
+			Perquintill::from_float(0.0250000249999995)
+		);
+	})
+}
+
 #[test]
 fn staking_should_work() {
 	ExtBuilder::default().nominate(false).build_and_execute(|| {